1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a technique for reducing heat generation of a semiconductor device to stabilize the operation thereof.
2. Description of the Background Art
FIG. 6 is a vertical cross-sectional view for illustrating a background art semiconductor device 101P. As shown in FIG. 6, the semiconductor device 101P comprises an Nxe2x88x92 type semiconductor substrate 1P made of silicon or the like, and a P layer 2P is formed in a surface S1P of the semiconductor substrate 1P. The P layer 2P is formed in contact with a beveled surface S2P (referred to also as a xe2x80x9csurface S2Pxe2x80x9dhereinafter) contiguous with the surface S1P. An anode electrode 31P is formed on the surface S1P. An N+ layer 25P is formed in a surface S3P opposed to the surfaces S1P and S2P. A cathode electrode 33P is formed on the surface S3P in opposed relation to the anode electrode 31P.
In general, carrier recombination centers are formed by proton irradiation or the like near a PN junction of the P layer 2P and the semiconductor substrate 1P, and control a carrier lifetime near the PN junction. The carrier lifetime in the entire semiconductor substrate 1P is controlled to be short by heavy metal diffusion, electron beam irradiation or the like.
FIG. 7 is a vertical cross-sectional view for illustrating current flows in the semiconductor device 101P. In FIG. 7, forward current is schematically shown by solid arrows and reverse current is schematically shown by dashed arrows.
FIG. 8 schematically illustrates a reverse recovery operation in the semiconductor device. In general, when switching of an external circuit causes the semiconductor device 101P to make an instantaneous transition from a forward current flowing state to a reverse-biased state, a large transient reverse current flows because of a minority carrier accumulation phenomenon. Such a reverse current has a current decrease rate determined by the value of the reverse bias and the inductance of the external circuit, and flows for a predetermined length of time.
More specifically, the reverse current flows until excess carriers remaining near the PN junction decrease in number to have a constant concentration or less and a depletion layer is established. When the established depletion layer starts bearing a reverse voltage, the reverse voltage gradually increases in accordance with the expansion of the depletion layer whereas the reverse current gradually decreases. Then, the device voltage becomes steadily equal to the applied reverse voltage, and the reverse recovery operation is completed.
In the background art semiconductor device 101P, the lifetime near the PN junction is controlled to be short as described above, thereby to decrease a forward voltage and the reverse current and to improve a maximum allowable on-state current rise rate (di/dt).
However, the background art semiconductor device 101P is disadvantageous in that the higher, for example, a driving frequency and the reverse bias voltage are, the greater a power loss, i.e. heat generation (self heat generation), is. Thus, the operation of the semiconductor device 101P might encounter a trouble due to the heat generation. At this time, the semiconductor substrate 1P generates a greater amount of heat in a peripheral portion 1AP thereof than near a central portion thereof, particularly a portion between the anode electrode 31P and the cathode electrode 33P. A cause of such a difference in the amount of heat generation is a poorer heat dissipating ability or heat emitting property of the peripheral portion 1AP of the semiconductor substrate 1P than that of the central portion thereof because of the absence of a metal layer such as the anode electrode 31P and the like on the surface of the peripheral portion 1AP of the semiconductor substrate 1P.
According to a first aspect of the present invention, a semiconductor device comprises: a first semiconductor layer of a first conductivity type having a first surface, a second surface surrounding the first surface and coupled to the first surface at an angle other than a right angle, and a third surface opposed to the first and second surfaces; a second semiconductor layer of a second conductivity type formed in the first surface; and a third semiconductor layer of the second conductivity type surrounding the second semiconductor layer in non-contacting relationship with the second semiconductor layer, the third semiconductor layer being formed in the first semiconductor layer and in contact with the second surface.
Preferably, according to a second aspect of the present invention, in the semiconductor device of the first aspect, a distance between the second semiconductor layer and the third semiconductor layer in their closely spaced positions is not greater than 50 xcexcm.
Preferably, according to a third aspect of the present invention, in the semiconductor device of the first or second aspect, the third semiconductor layer extends closer to the third surface than does the second semiconductor layer.
Preferably, according to a fourth aspect of the present invention, the semiconductor device of any one of the first to third aspects further comprises a metal layer formed on the side of the third surface to cover a region of the third surface which is opposed to the second semiconductor layer.
Preferably, according to a fifth aspect of the present invention, the semiconductor device of any one of the first to fourth aspects further comprises a fourth semiconductor layer of the first conductivity type formed in part of the third surface which is substantially opposed to the second semiconductor layer, wherein the fourth semiconductor layer has an impurity concentration higher than the impurity concentration of its surrounding layer.
Preferably, according to a sixth aspect of the present invention, the semiconductor device of any one of the first to fifth aspects further comprises a fifth semiconductor layer of the first conductivity type formed in contact with the third surface, the fifth semiconductor layer having an impurity concentration higher than the impurity concentration of the first semiconductor layer.
In accordance with the first aspect of the present invention, the third semiconductor layer surrounds the second semiconductor layer in non-contacting relationship with the second semiconductor layer, and is formed in the first semiconductor layer and in contact with the second surface. This allows current to converge on the second semiconductor layer and, therefore, on the center of the first semiconductor layer, thereby suppressing heat generation in a peripheral portion of the first semiconductor layer. Therefore, the semiconductor device having the second surface corresponding to a so-called beveled surface can suppress a trouble resulting from the heat generation to perform a stable operation. Additionally, the second surface ensures a breakdown voltage or maximum allowable voltage.
In accordance with the second aspect of the present invention, the distance between the second semiconductor layer and the third semiconductor layer in their closely spaced positions is not greater than 50 xcexcm. This ensures the suppression of a leakage current flowing when a reverse voltage is applied between the second semiconductor layer and the third surface.
In accordance with the third aspect of the present invention, the third semiconductor layer extends closer to the third surface than does the second semiconductor layer. This effectively alleviates a surface electric field strength at the second surface. Therefore, the increase in breakdown voltage of the semiconductor device is accomplished.
In accordance with the fourth aspect of the present invention, the metal layer is formed on the side of the third surface to cover the region of the third surface which is opposed to the second semiconductor layer. In other words, the metal layer covers, from the third surface side, a region in which converging current flows. Therefore, the metal (layer) having a great heat dissipating ability efficiently dissipates generated heat. The metal layer may cover a wider area including the above-mentioned region, thereby to further improve the ability to dissipate heat from the third surface.
In accordance with the fifth aspect of the present invention, the fourth semiconductor layer has the impurity concentration higher than the impurity concentration of its surrounding layer. Thus, the semiconductor device of the fifth aspect can decrease the reverse current during the reverse recovery operation, as compared with a semiconductor device which does not comprise the fourth semiconductor layer. Additionally, the fourth semiconductor layer formed in part of the third surface which is substantially opposed to the second semiconductor layer allows the reverse current to converge on the fourth semiconductor layer. This suppresses the heat generation in the peripheral portion of the first semiconductor layer during the reverse recovery operation. Therefore, the semiconductor device is provided which suppresses a trouble resulting from the heat generation to perform a stable operation.
In accordance with the sixth aspect of the present invention, the fifth semiconductor layer has the impurity concentration higher than the impurity concentration of the first semiconductor layer. Thus, the semiconductor device of the sixth aspect can decrease the reverse current, as compared with a semiconductor device which does not comprise the fifth semiconductor layer. This suppresses the heat generated by the reverse current. Therefore, the semiconductor device is provided which suppresses a trouble resulting from the heat generation to perform a stable operation.
Moreover, the fifth semiconductor layer formed in contact with the third surface can decrease an on-state voltage when an electrode is provided in contact with the third surface.
It is therefore a primary object of the present invention to provide a semiconductor device capable of suppressing heat generation to perform a stable operation.
It is another object of the present invention to provide a semiconductor device capable of ensuring the suppression of a leakage current flowing upon application of a reverse voltage, as well as accomplishing the primary object.
It is still another object of the present invention to provide a semiconductor device having a high breakdown voltage (or high maximum allowable voltage), as well as accomplishing the above-mentioned object(s).
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.